A Study On Rf Cavity Simulator And Self-Excited Loop System Of Low Level RF Control System

The shanghai high-repetition-rate X-ray free-electron laser and extreme light facility(SHINE)will be operated in 1 MHz repetition rate and will be offered electron source by Very High Frequency(VHF)gun and will be driven by 1.3-GHz continuouswave(CW)superconducting radio-frequency(RF)cavities.To obtain high quality electron beam,a high precision low-level RF(LLRF)system for a superconducting RF cavity is required to stabilize the amplitude and phase of the cavity field.However,during the CW conditioning or start-up process of the VHF gun,the deformation in geometry will make the electromagnetic resonant frequency of the cavity be quickly detuned dozens k Hz,which is caused by the power dissipation of the cavity wall.Mechanical tuning system of the VHF gun is difficult to tune in time.And,the frequent and long-distance movement will easily damage the mechanical tuner.Moreover,a larger cavity resonant frequency detuning will also cause a larger reflected power of the cavity,which makes it difficult for the cavity to be fed microwave power.In addition,in the 1.3 GHz CW superconducting RF cavity,the cavity has a large load quality factor,which results in a very narrow bandwidth of the cavity in the frequency domain.It makes the small cavity detuning will cause a large reflected power,triggering the interlock.Besides,the electromagnetic field in the cavity and the current of the cavity well will cause lorentz force detuning(LFD)effect.During the conditioning or start-up process of the superconducting cavity,the electromagnetic mode in the superconducting cavity will interact with the mechanical mode,causing "ponderomotive" instability in the control system,making it difficult for microwave power to feed into the cavity.Therefore,a self-excited loop(SEL)LLRF control system that can be used in the conditioning or start-up process is designed and developed in this paper.By adjusting the phase and gain in the loop,the loop frequency of the control system can follow the cavity resonance frequency in real time.Furthermore,such a control system is steady and the cavity field is stable.The SEL control system not only avoids the frequent movement of the mechanical adjustment system of the VHF electron gun,but also prevents the "ponderomotive" instability of the 1.3 GHz CW RF cavity,and the reflected power of the cavity is also low.At the same time,the Generator Driven Resonator(GDR)control system also be developed and used in the operation condition of the RF cavity.In addition,this paper also derives a multi-feed cavity model suitable for any RF cavity,which is used to study cavity characteristics and provide a theoretical basis for cavity measurement.Based on the cavity model,the digital cavity simulator of the VHF electron gun and 1.3 GHz CW superconducting cavity also be developed for researching and testing of the advanced algorithms of the LLRF system and.Based on the Micro TCA.4 platform,the laboratory testing of the LLRF system of the 1.3 GHz CW superconducting cavity and the digital cavity simulator also be completed in the paper.The functions and algorithms of the developed LLRF control system are verified.After comparison and validation,the cavity simulator was verified to be a reliable platform to test the new algorithms.